Datasheet

MAX1836/MAX1837 Stability
Commonly, instability is caused by excessive noise on the
feedback signal or ground due to poor layout or improper
component selection. When seen, instability typically
manifests itself as “motorboating,” which is characterized
by grouped switching pulses with large gaps and exces-
sive low-frequency output ripple during no-load or light-
load conditions.
PC Board Layout and Grounding
High switching frequencies and large peak currents make
PC board layout an important part of the design. Poor lay-
out may introduce switching noise into the feedback path,
resulting in jitter, instability, or degraded performance.
High-power traces, bolded in the typical application cir-
cuits (Figure 1 and Figure 2), should be as short and wide
as possible. Additionally, the current loops formed by the
power components (C
IN
, C
OUT
, L1, and D1) should be
as tight as possible to avoid radiated noise. Connect the
ground pins of these power components at a common
node in a star-ground configuration. Separate the noisy
traces, such as the LX node, from the feedback network
with grounded copper. Furthermore, keep the extra cop-
per on the board, and integrate it into a pseudoground
plane. When using external feedback, place the resistors
as close to the feedback pin as possible to minimize noise
coupling. The MAX1837 evaluation kit shows the recom-
mended layout.
Applications Information
High-Voltage Step-Down Converter
The typical application circuits’ (Figure 1 and Figure 2)
components were selected for 9V battery applications.
However, the MAX1836/MAX1837 input voltage range
allows supply voltages up to 24V. Figure 6 shows a modi-
fied application circuit for high-voltage applications. When
using higher input voltages, verify that the input capaci-
tor’s voltage rating exceeds V
IN(MAX)
and that the induc-
tor value exceeds the minimum inductance recommended
in the Inductor Selection section.
Inverter Conguration
Figure 7 shows the MAX1836/MAX1837 in a floating
ground configuration. By connecting what would nor-
mally be the output to the supply-voltage ground, the IC’s
ground pin is forced to regulate to -5V (MAX183_EUT50)
or -3.3V (MAX183_EUT33). Avoid exceeding the maxi-
mum ratings of 24V between IN and GND, and 5.5V
between OUT and GND. Other negative voltages may be
generated by placing a resistive divider across the output
capacitor and connecting the tap to FB in the same man-
ner as the normal step-down configuration.
Table 1. Component Suppliers
SUPPLIER PHONE FAX WEBSITE
INDUCTORS
Coilcraft 847-639-6400 847-639-1469 www.coilcraft.com
Coiltronics 561-241-7876 561-241-9339 www.coiltronics.com
Sumida USA 847-956-0666 847-956-0702 www.sumida.com
Toko 847-297-0070 847-699-1194 www.tokoam.com
CAPACITORS
AVX 803-946-0690 803-626-3123 www.avxcorp.com
Kemet 408-986-0424 408-986-1442 www.kemet.com
Panasonic 847-468-5624 847-468-5815 www.panasonic.com
Sanyo 619-661-6835 619-661-1055 www.secc.co.jp
Taiyo Yuden 408-573-4150 408-573-4159 www.t-yuden.com
DIODES
Central Semiconductor 516-435-1110 516-435-1824 www.centralsemi.com
International Rectier 310-322-3331 310-322-3332 www.irf.com
Nihon 847-843-7500 847-843-2798 www.niec.co.jp
On Semiconductor 602-303-5454 602-994-6430 www.onsemi.com
Zetex 516-543-7100 516-864-7630 www.zetex.com
MAX1836/MAX1837 24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
www.maximintegrated.com
Maxim Integrated │ 11